The present invention relates generally to apparatus and methods for impact blasting lithographic sheet material and the like with a very fine, lightweight blasting media to provide a fine-grained, uniformly etched finish.
Metal sheets used in lithographic printing processes require finely textured, uniformly etched surfaces. A preferred material for use in lithography is aluminum and alloys thereof, but other materials may also be used. Surface etching of lithographic sheet material is normally conducted using chemical etching techniques. Chemical etching is effected by applying one or more chemicals to the surface to be etched. A chemical reaction results which causes portions of the exposed metal surface to be eaten away, thereby creating a uniform array of fine surface indentations which give the resulting product a fine-grained, satin-like finish.
While chemicl etching techniques are capable of providing suitably uniform finishes, these techniques are expensive to perform. Moreover, the resulting spent chemicals include hazardous substances which present environmental disposal problems. In view of these drawbacks, industry has long sought a suitable, less expensive, non-polluting method of uniformly etching the surfaces of metal sheets for use in lithography.
The type of etched finish required for use in lithography is quite different from the type of surface treatment normally obtained using impact blasting techniques. If an etched finish is to be acceptable for lithographic purposes, it must be quite smooth, typically about as smooth as the types of surfaces which result from some honing, cold rolling, drawing or grinding operations. In many instances, the surface must be as smooth as that which is achieved with drilling or milling operations. Moreover, the etched surface profile must be held to what is known in the art as a specified "AA" etch number, with a tolerance of only plus or minus 3AA over the entire etched surface. Typically, the specified "AA" number is within the range of about 17AA to about 40AA.
Impact blasting techniques previously have been found to be incapable of providing the type of etched surface required for lithographic purposes. The etching surface consistency required for lithographic sheets is far more stringent than that obtainable through the use of conventionl impact blasting techniques.